The use of gene expression profiling is not only prevalent in various research applications, but is rapidly becoming part of many therapeutic regimes. For example, the determination of gene expression levels in tissues is of great importance for accurately diagnosing human disease and is increasingly used to determine a patient's course of treatment. Pharmacogenomic methods can identify patients likely to respond to a particular drug and can lead the way to new therapeutic approaches.
For example, thymidylate synthase (TS) is an integral enzyme in DNA biosynthesis where it catalyzes the reductive methylation of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP) and provides a route for de novo synthesis of pyrimidine nucleotides within the cell (Johnston et al. (1995) Cancer Res., 55: 1407-1412). Thymidylate synthase is a target for chemotherapeutic drugs, most commonly the antifolate agent 5-fluorouracil (5-FU). As an effective agent for the treatment of colon, head and neck and breast cancers, it is believed the primary action of 5-FU is to inhibit TS activity, resulting in depletion of intracellular thymine levels and subsequently leading to cell death.
Thymidylate synthase is also known to have clinical importance in the development of tumor resistance, as demonstrated by studies that have shown acute induction of TS protein and an increase in TS enzyme levels in neoplastic cells after exposure to 5-FU (Spears et al. (1982) Cancer Res. 42: 450-456; Swain et al. (1989) J. Clin. Oncol. 7: 890-899). The ability of a tumor to acutely overexpress TS in response to cytotoxic agents such as 5-FU may play a role in the development of fluorouracil resistance. The levels of TS protein appear to directly correlate with the effectiveness of 5-FU therapy, that there is a direct correlation between protein and RNA expression and TS expression is a powerful prognostic marker in colorectal and breast cancer (Jackman et al. (1985) Experimental and Clinical Progress in Cancer Chemotherapy, F. M. Muggia ed., Martinus et al. (1992) Cancer Res., 52: 108-116). In advanced metastatic disease, both high TS mRNA, quantified by RT-PCR, and high TS protein expression, have been shown to predict a poor response to fluoropyrimidine-based therapy for colorectal (Johnston et al. (1995) supra.; Farrugia et al. (1997) Proc. Annu. Meet Am. Assoc. Cancer Res. 38: A4132; Leichman et al. (1997) J. Clin. Oncol. 15(10): 3223-3229), gastric (Lenz et al. (1998) Clin. Cancer Res., 4(5): 1227-1234), and head and neck (Johnston et al. (1995)Cancer Res., 55: 1407-1412; Leichman et al. (1997) J. Clin. Oncol. 15(10): 3223-3229) cancers.
Similarly, mutation of the KRAS oncogene is predictive of a very poor response to panitumumab (VECTIBIX®) and cetuximab (ERBITUX®) therapy in colorectal cancer (Lièvre et al. (2006) Cancer Res., 66(8): 3992-3995). Currently, one of the most reliable ways to predict whether a colorectal cancer patient will respond to one of the EGFR-inhibiting drugs is to test for certain “activating” mutations in the gene that encodes KRAS, which occur in 40% of colorectal cancers. Studies show patients whose tumors express the mutated version of the KRAS gene will not respond to cetuximab or panitumumab.
One important source for this type of information comes in the form of formalin-fixed, paraffin-embedded tissue (“FFPET”) samples, that are routinely created from biopsy specimens taken from patients undergoing a variety of diagnostic and/or therapeutic regimens for a variety of different diseases. These samples are usually associated with the corresponding clinical records and often play an important role in diagnosis and determination of treatment modality. For example, tumor biopsy FFPET samples are often linked with cancer stage classification, patient survival, and treatment regime, thereby providing a potential wealth of information that can be cross-referenced and correlated with gene expression patterns. However, the poor quality and quantity of nucleic acids isolated from FFPET samples has led to their underutilization in gene expression profiling studies.
It is known that RNA can be purified and analyzed from FFPET samples (Rupp and Locker (1988) Biotechniques 6: 56-60), however, RNA isolated from FFPET samples is often moderately to highly degraded and fragmented. In addition to being degraded and fragmented, chemical modification of RNA by formalin restricts the binding of oligo-dT primers to the polyadenylic acid tail and can impede the efficiency of reverse transcription.
In view of these difficulties, the analysis of nucleic acids from formalin fixed, paraffin embedded tissue (FFPET) has proven challenging due to the multiple steps required for generating PCR-amplifiable genetic material. The procedure to isolate nucleic acids from FFPET may include; removal of paraffin (deparaffinization), lysis of preserved sample (protease digestion), reversal of cross-links acquired during the fixation process and solid phase-based purification of nucleic acids. These protocols are typically complex and labor intensive.